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Natural Gas Industry B 1 (2014) 221e229www.elsevier.com/locate/ngib

Research article

Significant breakthrough in proprietary deepwater natural gas exploration inthe northern South China Sea and its inspiration*

Xie Yuhong

CNOOC Ltd, Zhanjiang, Guangdong 524057, China

Received 8 September 2014; accepted 25 October 2014

Available online 7 April 2015

Abstract

In 2014, Zhanjiang Branch of CNOOC Limited obtained high oil and gas flows (160 � 104 m3/d) in Well LS 17-2-1 in northern South ChinaSea. The tested formation is the Neogene Upper Miocene Huangliu Fm. This is the first self-explored natural gas field in deep water area inChina. Exploration in this area underwent three periods: (1) Reconnaissance exploration period (before 2002): Limited by techniques, the majorexploration was in shallow water areas; (2) Foreign cooperative exploration period (2002e2012): As the discovered reserve scale was small, andthere is no economic benefit, the partners successively gave up their exploration equities in the deep water area of western South China Sea;however, the high quality source rocks and reservoirs were confirmed in the deep water area of the South China Sea, revealing the mask ofhydrocarbon exploration in this region; (3) Self-exploration period (since 2013): Relying on national science and technology major projects andits scientific research and production, Zhanjiang Company of CNOOC conducted several scientific researches on seismic data acquisition andprocessing, high-quality reservoir distribution, key factors for hydrocarbon accumulation in deep water areas, optimized and confirmed Lingshui17-2 structure as the first drilling target, and obtained significant exploration breakthrough. Lingshui 17-2 structure is located in the Ledong-Lingshui section of the Central Canyon. Its major target layer is the Huangliu Fm. Its total thickness is over 150 m, with sandstone ratio of75.4%, maximum single layer thickness of 52 m, porosity ranging 30.0%e33.7% (31.5% averagely), permeability ranging 293e2512 mD (633mD averagely), belonging to reservoir with ultrahigh porosity and high-ultrahigh permeability. The exploration breakthrough in Lingshui 17-2shows the giant potentiality of hydrocarbon exploration in this deep water area.© 2015 Sichuan Petroleum Administration. Production and hosting by Elsevier B.V. This is an open access article under the CC BY-NC-NDlicense (http://creativecommons.org/licenses/by-nc-nd/3.0/).

Keywords: Qiongdongnan basin; Deep water; Central canyon; Lingshui 17-2 gas reservoir; Neogene; Natural gas; Exploration; Significant breakthrough;

Inspiration

1. Geologic background

The Qiongdongnan Basin is located at the continentalmargin of northern South China Sea. It is a Cenozoic rift-depression basin, with tectonic features of quasi-passive con-tinental margin basins. On plane, it shows a zoning featurefrom the north to the south, and is divided into such four

* Fund project: Special and Significant Project of National Science and

Technology for the “12th Five-year Plan” “Evaluation of potential hydrocar-

bon generation sags in Northern South China Sea” (No. 2011ZX05025-002).

E-mail address: zhoujx@cnooc.com.cn.

Peer review under responsibility of Sichuan Petroleum Administration.

http://dx.doi.org/10.1016/j.ngib.2014.11.015

2352-8540/© 2015 Sichuan Petroleum Administration. Production and hosting by

(http://creativecommons.org/licenses/by-nc-nd/3.0/).

tectonic units as Northern Depression, Middle Uplift, CentralDepression and Southern Uplift (Fig. 1). This basin underwentan evolution from continent to marine at four stages: riftingstage (T100eT70), fault-depression stage (T70eT60), thermalsubsidence stage (T60eT30) and accelerated subsidence stage(T30-present), which can be divided into two major periods:chasmic period (T100eT60) and post-chasmic period (T60-pre-sent). Its absolute geologic ages are shown in Fig. 1. Depositedin this basin are super-thick Cenozoic formations with thick-ness >12 000 m [1e3].

Based on seismic data features, most of the faulting ac-tivities in the deep water area were in Eocene Epoch, Oligo-cene Yacheng stage and early Lingshui stage, gradually

Elsevier B.V. This is an open access article under the CC BY-NC-ND license

Fig. 1. Tectonic unit division and location of central canyon channel in the Qiongdongnan Basin.

222 Y. Xie / Natural Gas Industry B 1 (2014) 221e229

weakened to late Lingshui stage, and progressively stoppedduring the third stage of Miocene Epoch. Hence, during theinitial development stage of this basin, faulting activitiesapparently controlled the deposition in this basin, where twosuites of important source rocks were developed. DuringEocene Epoch, faulting and extensional activities were violent,forming a morphologic framework featured by multi-sag andmulti-salient. In sag areas, middle-deep lacustrine depositionalsystems were developed, and thicker Eocene lacustrine sourcerocks were deposited [4], with a thickness predicted as up to3000 m maximally. These source rocks have entered a matureto over-mature stage, with bigger hydrocarbon generation

potentiality, being the latent favorable source rocks. DuringOligocene Yacheng stage, this basin gradually changed totransitional and neritic settings. This was the key developingstage of the major source rocks, forming two sets of sourcerocks (neritic mudstone, coastal plain coal-measuremudstone). Neritic mudstone predominates in the centralarea of sags, whereas in the periphery of them and some highpositions, coastal plain sedimentary features predominate,accompanied by the development of coal-measure formations[4,5]. This set of source rocks have been identified by drillingto be better ones. The distribution area of Oligocene YachengFm is obviously bigger than that of Eocene series. Yacheng

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Fm is about 2100 m thick in the subsiding center. Based onbasin modeling results, the major part of Yacheng Fm hasreached mature stage of hydrocarbon expulsion, with hugehydrocarbon generation potentiality.

The Qiongdongnan Basin underwent geologic evolutionfrom continent to marine, accompanied by the development ofvarious reservoir bodies like channel sandstones, low fans,subsea aprons, organic reefs, offshore sand bars, deltas andburied hills. Among them, the most apparent one is thechannel sandstones developed in the Central Canyon in theCentral Depression Zone during the Miocene Huangliu stage.The west edge of the Central Canyon is at the eastern slope ofthe Yinggehai Basin, and its east edge is in the northwesternsub-ocean basin of the South China Sea, with a total length of425 km and a maximum width of 48.5 km. It generallystretches in SWeNE direction, and distributes in “S” shape onplane. It also has “sectioning” features, and can be divided intoLedong section, Lingshui section, Songnan-Baodao sectionand Changchang section from the west to the east (Fig. 1); onsections, it shows symmetrical or asymmetrical “V”, “U” and“W” shapes (Fig. 2) [6,7]. Controlled by many geologic fac-tors, the internal packing structure inside the Central Canyonhas a multiphasic feature, as a result, it was filled by severalsets of favorable overlapped reservoir-caprock assemblages ofsandstone and mudstone; with the help of lateral sealing of thecanyon edges and lithologic pinching out, several structuraland lithologic traps were formed, showing great explorationpotentialities.

2. Discovery of Lingshui 17-2 gas reservoir

2.1. Basic status

Lingshui 17-2 structure is located in the Ledong-Lingshuisection of the Central Canyon, about 150 km away from theHainan Island (Fig. 1). Well LS 17-2-1 was spud in on January10, 2014, completed on February 9, with a total depth of3510 m, and the finally drilled formation was the NeogeneUpper Miocene Huangliu Fm.

Mud logging showed gas testing anomalies in the 2ndmember of Pliocene Yinggehai Fm and the 1st member ofUpper Miocene Huangliu Fm, with a length of 23 m and 64 mrespectively.

On August 18, 2014, well test was carried out in3321e3351 m interval in the 1st member of Neogene UpperMiocene Huangliu Fm, 160 � 104 m3 gas and 78 m3

condensate oil were produced per day by 25.4 mm choke.Considering the offshore safety, no further well test wasconducted by bigger chokes. Well test results showed that thegas pressure was stable and the producing energy wassufficient.

2.2. Geologic features

2.2.1. Formations and pay zonesRevealed by Well LS 17-2-1, the formations from top to

bottom are Pleistocene Ledong Fm, Pliocene Yinggehai Fm

and Upper Miocene Huangliu Fm (not penetrated) in turn, andthe Huangliu Fm is the key target zone. Drilling results showcomplete developed formations. The Ledong Fm is mainlycomposed of mudstone in deep marine, with silty mudstone atits bottom. The 1st member of the Yinggehai Fm is stilldominantly mudstone as a whole, with silty mudstone locally;in the 2nd member, sandstone content begins to increase, andthick argillaceous siltstones emerge in its middle intervals,with a maximum thickness of 32 m and a mud ratio of 86.9%.As a major exploration target zone, the Huangliu Fm wasencountered by several sets of thick sandstones, with a totalthickness of 158.5 m and a sandstone ratio of 75.4%; themaximum thickness of single sandstone layer is obviouslybigger than that of the 2nd member of the Yinggehai Fm(maximum thickness of up to 52 m); the grain size of sand-stone becomes bigger, mainly packsand and siltstone-packsand, exhibiting continuous strong amplitude reflectionson seismic profiles. By well logging interpretation, the reser-voir is 131.3 m thick. The porosity of coring intervals ranges30.0%e33.7% (31.5% averagely), and the permeability ranges293e2512 mD (633 mD averagely), indicating that it belongsto a reservoir with ultrahigh porosity and high-ultrahighpermeability (Fig. 3).

2.2.2. Gas reservoir featuresGas components show that heavy hydrocarbon content in

the Lingshui 17-2 gas reservoir are low (0.26%), with a rela-tive density ranging 0.613e0.623; methane content ranges91.77%e92.77%, with an aridity coefficient of 0.95. It wasinferred that its source was the coastal plain coal-measuresource rocks of the Yacheng Fm.

In Lingshui 17-2 gas reservoir, the temperature ranges70.71e79.43 �C, the geothermal gradient is 3.79 �C/100 m,belonging to a normal temperature system; the reservoirpressure ranges 39.03e39.38 MPa, the pressure coefficient is1.2, belonging to a normal pressure system.

3. Exploration history

3.1. Reconnaissance exploration period (before 2002)

Limited by technique bottlenecks, exploration was mainlyconducted in the shallow water area of the QiongdongnanBasin for a long time. But several medium- and large-scale oiland gas fields were discovered. With the increase of explora-tion degree, the exploration in the shallow water area gradu-ally entered a mature stage. According to global oil and gasexploration discoveries, 70% hydrocarbon resources werestored in deep seas. Therefore, striding to deep water naturalgas exploration has become our strategic policy for mediumand long-term development.

The seismic data were mainly acquired in the shallow waterarea to the north of Central Depression of the QiongdongnanBasin before 2002. The Central Depression and the region toits south cover a 53 000 km2 deep water area, but seismic dataacquisition was conducted in a small area of them. Theseismic data related to the deep water area acquired during this

Fig. 2. “U”, “V” and “W” shaped profiles of the Central Canyon.

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period were some 2D seismic data acquired for understandingthe regional tectonic features of the Qiongdongnan Basin,which were acquired in 1979, 1980, 1987, 1991, 1992, 1994and 1997 respectively. The total length of these 2D seismicdata was about 9000 km, with a survey grid density of15 km � 18 km in most regions (even 36 km � 36 km or36 km � 18 km in some regions), thus the seismic data were

scarce. The cable length of the seismic receiving system wasshort (less than 5000 m), with little seismic source energy andshort seismic recording time (less than 8 s, only 5 s in theChangchang area). Restricted by seismic acquisition andprocessing techniques, the overall quality of seismic data waspoor, with low signal/noise ratio, indistinct composite wavereflection signatures, unsharp relationship between sag-

Fig. 3. Composite columnar section of Huangliu Fm in Well LS 17-2-1.

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controlled faults and formations, and short seismic recordingtime for the deep formations of the sags, and the depositionalbasements of some sags could not be confirmed. Moreover,during seismic data acquisition, small-scale offshore gravityand magnetic reconnaissance and general survey were alsoconducted in the whole basin.

For a long time, the regional geology of the deep water areawas studied mainly relying on the national “9th Five-yearPlan” key research programs. Hence, some preliminary re-searches on regional geotectonic backgrounds, filling se-quences, basinal structures, tectonic evolution andhydrocarbon geologic features were only conducted. Based onthis, six hydrocarbon generation sags like Ledong, Lingshui,Songnan, Baodao, Changchang and Beijiao and two positiveunits like Lingnan Low Salient and Songnan Low Salient wereclassified in the deep water area of the Qiongdongnan Basin.Hydrocarbon resources of these six sags were initially evalu-ated, with natural gas resources predicted as 2 � 1012 m3,showing huge hydrocarbon generation potentiality in the deepwater area. Generally speaking, as there were only a fewseismic data, and exploration research during this periodfocused only on some regional studies for exploring theshallow water area, the research degree of petroleum geologyfeatures of the deep water area was very low.

As for drilling, only Well BD 23-1-1, the first well withwater depth exceeding 300 m (352 m actually) was drilled inthe Shenhu Uplift in 1993 without any hydrocarbon discovery.

3.2. Foreign cooperative exploration period(2002e2012)

Deep water drilling was characterized by “high risk, highcost and high return”. The expense for drilling an exploratorywell in the area with water depth of more than 1000 m wasabout $100 million, which was about 7e10 times that ofdrilling in shallow water area. From the view of global deepwater exploration, mutual equity participation by several oilcompanies was the major mode for deep water exploration,which could reduce both risk and cost. Under the situation oflow research degree on hydrocarbon accumulation, immaturedrilling techniques and expensive exploration cost in deepwater exploration, foreign cooperation was a favorable choicefor quickening deep water exploration. By means of foreigncooperation, CNOOC could obtain a great deal of 2D and 3Dseismic data, confirm the exploration potentiality of the basinand sags, and optimize plays and favorable targets. Using theglobal advanced techniques in deep water exploration forreference, not only the progress of deep water explorationcould be promoted, but also the breakthrough in deep waterexploration could be made as soon as possible.

3.2.1. The failure stage of initially exploring deep waterthrough foreign cooperation (2002e2004)

The major problems during this stage included: lack ofseismic data of deep water area, poor quality of them, un-known basic conditions for hydrocarbon accumulation;absence of real geologic data of deep water drilling, lowexploration degree, and no completely mastered deep waterdrilling techniques. Restricted by technical conditions, avail-able data and exploration investment, the main explorationstrategy during this stage was to, through cooperation withforeign companies, follow the global deep water explorationsteps, introduce foreign capital to reduce the exploration risk,study advanced deep water exploration and appraisal tech-niques of partners, especially the drilling techniques, so as topromote the deep water exploration progress in South ChinaSea. From this stage on, 2D seismic data were acquired forexploring the oil and gas in the deep water area of theQiongdongnan Basin. Some studies on prospecting the sagsand the basin, sag framework and regional sedimentation wereconducted, and the oil and gas resources in the deep water areaof western South China Sea were preliminarily evaluated.

Since 2002, investment promotion had been commencedfor deep water exploration. Seven blocks in the deep waterarea and the Shenhu Uplift in the Qiongdongnan Basin werereleased for bidding. In 2003, H Company signed a contractwith CNOOC to explore deep water oil in the Shenhu Uplift.This block is located in the Shunde Sag, which is to the southof the Shenhu Uplift, Zhuer Depression, western Pear RiverMouth Basin, with water depth of 300e2000 m. Afterappraisal, H Company began to drill a well in Changchang 12-

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1 structure of this block in 2004. This is the first well drilled inwater depth of more than 500 m in this block. It is also a realdeep water exploratory well. The drilled well confirmed thereservoir-caprock assemblage of deltaic-littoral reservoirs inthe Zhujiang Fm and Zhuhai Fm and the overlying neriticmudstones. The reservoir properties are moderate to good, butnot any commercial hydrocarbon reservoir was discovered.

3.2.2. The early stage of cooperative exploration climax(2004e2007)

During this stage, Zhanjiang Company of CNOOC (here-after called “Zhanjiang Company”) mainly solved the keyproblems like indistinct hydrocarbon exploration potentialityand unknown favorable exploration domains and directions ofthe deep water sags. In the process of solving the aforesaidproblems, Zhanjiang Company specially studied and devel-oped seismic acquisition and processing techniques such aslong seismic cable (10 124 km long), upper and lower cables,and upper and lower seismic sources. Combining with thenational science and technology key programs of the “10thFive-year Plan”, Zhanjiang Company conducted studies onbasic conditions of hydrocarbon accumulation, sag evaluationand favorable play optimization of the deep water area, andthen strengthened the release of blocks for foreigncooperation.

The researches during this stage deepened our under-standing of the petroleum systems in the Central Depression.Research suggests that the Qiongdongnan Basin is a Cenozoicrift-depression basin at the northern continental margin of theSouth China Sea, characterized by “rift in the lower part anddepression in the upper part as well as multi-salients andmulti-sags”. The interaction between the Pacific Plate and theEurasian Plate and the sea floor spreading of the South ChinaSea are the major factors controlling the formation anddevelopment of the Qiongdongnan Basin. Several sets ofsource rocks like Eocene lacustrine facies, Oligocene YachengFm marine facies and transitional facies were developed, withnatural gas resource potentiality of about 4 � 1012 m3. Con-tinental slope started to develop in the Qiongdongnan BasinSince Miocene, and the Central Depression gradually entereda deep water depositional environment of continental slope,where gravity flow deposits such as subsea aprons and turbi-dite channels were developed. The research results also showthat the Central Depression has favorable hydrocarbon accu-mulation conditions, exhibiting a high thermal evolution de-gree of source rocks, large hydrocarbon generationpotentiality, various favorable reservoir types, numerousreservoir bodies and it is located in the major hydrocarbonmigration and accumulation directions. Based on the aboveunderstandings, some favorable plays like draping anticlinestructural zones in Lingnan Low Uplift, Songnan Low Upliftand Yongle Low Uplift were screened out priliminarily forclassifying blocks for foreign bidding.

Attracked by global deep water exploration upsurge andhuge potentiality in the deep water area of the South ChinaSea, contracts of four cooperative exploration blocks in thedeep water area of the western South China Sea were signed in

2006 and 2007 respectively. Specifically, B Company signed acontract for one block in Songnan Low Uplift and another inLingshui Sag with CNOOC; and C Company signed one forone block in Changchang Sag and another in Beijiao Sag withCNOOC. The realization of foreign cooperation in theseblocks marked the coming of foreign cooperative explorationclimax in the deep water area.

3.2.3. The parallel study stage of foreign cooperativeexploration and self-exploration (2007e2012)

During this stage, 2D seismic data (18 592 km) and 3Dseismic data (7931 km2) were acquired in the deep water areaof the Qiongdongnan Basin. In the meantime, ZhanjiangCompany formally established a study team for deep waterexploration project. Combined with the special and significantproject of national science and technology for the “11th Five-year Plan” and the “12th Five-year Plan” like “The keytechnology of deep water exploration in South China Sea” and“The key technology of deep water oil and gas exploration”,three levels (national research projects, CNOOC projects andproduction projects of Zhanjiang Company) of researcheswere formed; and simultaneously, foreign cooperation studieswere also further strengthened.

The researches were mainly conducted for some potentialhydrocarbon-rich sags such as the Lingshui Sag, the BaodaoSag and the Changchang Sag as well as the favorable zonesand targets around them, and confirmed some favorable playslike lithologic-structural trap zone in the Central Canyon,draping anticline structural zone in the Lingnan Low Salientand uplift structural zone in the Changchang Sag. Severaltargets like Lingshui 22-1, Lingshui 33-1, Changchang 33-1,Changchang 26-1, Yongle 19-1, Yongle 7-1, Yongle 8-2 andYongle 2-1 were evaluated, and drilling proposals were putforward for all of them.

Under the joint efforts of CNOOC and foreign partners, BCompany and C Company drilled a total of five exploratorywells in three sags and two low salients in the deep water areain 2010e2012, confirmed the development of Yacheng Fmcoal-measure source rocks, Upper Miocene Huangliu Fmturbidite channel sandstone and Miocene Sanya Fm subseaapron high-quality reservoirs in the deep water area, estab-lished the hydrocarbon accumulation mode of the CentralCanyon, and disclosed a corner of exploration potentiality inthe Lingshui Sag.

During this stage, although great progress was made inhydrocarbon exploration understandings and drilling tech-niques, because of small-scale discovered reserve, lack ofdevelopment economy, partners successively gave up theirexploration equities in the deep water area of western SouthChina Sea. Foreign cooperative exploration in deep water areawas quickly changed from upsurge to trough.

3.3. Self-exploration period (2013e2014)

After several global famous oil companies gave up theirexploration equities, Zhanjiang Company started their way toself-exploration in deep water area and greeted opportunities

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in the challenge. The exploration and development team ofZhanjiang Company accepted the challenge, used the self-developed seismic acquisition design technique, the core ofwhich is based on ray tracing, analysis of wave field lightingand forward simulation of wave equation, to greatly enrich thedata base for deep water natural gas exploration research. Atthe same time, the team developed an imaging technique thatis based on multi-domain association amplitude preservingand muting, integrated high-fidelity noise removal (LIFT) andplane wave migration to establish a high-accuracy seismic dataprocessing technique suitable for complex sea bed topographyand geologic structural features, which greatly increased thesignal/noise ratio of seismic data, widened the effectivebandwidth of seismic data, and improved the resolution ofseismic data. After thorough post-drilling analysis having beenconducted on the five exploratory wells drilled in the deepwater area, it was believed that hydrocarbon sources, reser-voirs and hydrocarbon migration are the key factors for hy-drocarbon accumulation in the deep water area.Simultaneously, relying on the special and significant projectof national science and technology for the “12th Five-yearPlan”, researches were conducted on three bottleneck prob-lems, confirmed huge potentiality of hydrocarbon resources,well developed high-quality and large-scale reservoir bodiesand clustering large-scale exploration targets in the deep waterarea, indicating that the deep water area has favorable pros-pects for hydrocarbon exploration.

3.3.1. Self denial to make a breakthrough in currenthydrocarbon generation theory

Hydrocarbon sources are the material bases for hydrocar-bon accumulation in traps. It was believed by some expertsboth at home and abroad that the source rocks in the deepwater area of western South China Sea were buried too deep,which passed the stage of hydrocarbon generation and ex-plosion in a great quantity, with probability of exhausted hy-drocarbon generation capability, thus having lost explorationvalue. Considering the universal overpressure existed in thedeep water sags and the effect of them to the hydrocarbongeneration in source rocks, the exploration research teamcarried out gas generation modeling experiments on coalsamples taken from the Yacheng Fm. They obtained a newunderstanding that high temperature and high pressure wouldsuppress hydrocarbon generation during an early period andpromote hydrocarbon generation during a late period, whichwidened the hydrocarbon generation scope of the sourcerocks. Based on this new understanding, a reasonable thermalhistory model was established to conduct basin modeling. Itwas discovered that the major parts of these sags in the deepwater area are still in the main gas generation window, and thesource rocks are generating gas in a great quantity, indicating agood hydrocarbon generation condition.

3.3.2. Innovating understandings to hunt for various typesof reservoirs

Reservoirs are one of the key factors for hydrocarbon accu-mulation in traps. The buried depth of strata in the deep water

area of western South China Sea is big; the physical propertiesof Paleogene target zones are poor; Neogene water depth is big,lack of provenance; there are no big rivers around this basin, andit is hard for large-scale high-quality reservoir bodies like deltasto form, therefore, the reservoir developing conditions seem tobe deficient congenitally, which was enhanced by the fact thatnot any high-quality reservoirs were encountered in lots ofwells. Based on the global deep water exploration practices, theexploration research team recognized that the deep water areabelow the basin slope break is the ideal place for developinggravity flow reservoirs, and thought that reservoir confirmationshould be changed from hunting for conventional Paleogenedeltas and littoral reservoirs to Neogene gravity flow reservoirs.Guided by the gravity flow theory, the exploration research teamused macro and micro provenance analysis technique and 3Dvisualization reservoir sedimentary facies study techniques toconduct integrated analysis, confirmed several favorable reser-voir bodies like turbidite channel sandstones and subsea apronsin the Central Canyon, and pointed out that the Central Canyonhas a sedimentary feature of sectional development, multiphasicfilling and multiple provenances, and is the most favorable re-gion for the development of reservoirs in the deep water area.

3.3.3. Using ant volume tracking technology to hunt forfaults and fractures

The conduction system is the bridge between hydrocarbonsource and reservoir bodies. In the deep water area, theNeogene reservoir bodies are seperated from the Paleogenehydrocarbon source by an argillaceous interval about 1000 mthick, so it is difficult for hydrocarbon to migrate. The re-searchers recognized that neo-tectonic movement occured inthe deep water area during a late period, and resulted in theformation of some late faults, micro-fractures and diapirs.Therefore, the researchers used the ant volume trackingtechnology to conduct careful and complete searching, sys-tematically analyzed the developing situation and scales offaults and diapirs as well as the connectivity between sandbodies, and finally confirmed the conduction system in thedeep water area.

3.3.4. Establishing hydrocarbon accumulation mode andobtaining great discoveries in drilling

With the help of high-precision seismic data and on thebasis of fine studies, the researchers proposed a new under-standing that hydrocarbons were accumulated in multiple setsof vertically overlapped sand bodies, which replaced the oldthought that hydrocarbons were accumulated in a single layerat the top of the Central Canyon. They established the hy-drocarbon accumulation mode of the deep water area: “hy-drocarbons were generated in older formations and stored innewer formations, migrated in vertical direction, accumulatedin gravity flow reservoirs, and sealed by bathyal mudstones”.In the deep water area of western South China Sea, wheremany difficulties in exploration were encountered at thebeginning, 14 drilling targets with well matched hydrocarbonsources, reservoirs and migration conditions were found,leading to the drilling of Lingshui 17-2 structure firstly.

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Up to now, the first four wells all have been successfullydrilled in the Lingshui 17-2 structure, proving that the Ling-shui 17-2 structure is a giant oil and gas field with high qualityand high production, and it is hopeful to build the first self-operated field of CNOOC in the deep water area during the“13th Five-year Plan”.

4. Exploration potentiality of the central canyon

4.1. Exploration prospects

In the deep water area of the Central Canyon in theQiongdongnan Basin, sources, reservoirs and caprocks arewell matched in time and space. Reservoir properties are su-perior. High flow rates were obtained in well tests. Lithologictraps are developed in groups. On balance, the potentiality inthis area is large. so it is a mature exploration domain forexpanding reserves in the deep water area for the moment.

The west edge of the Central Canyon is at the eastern slopeof the Yinggehai Basin, and its east edge is in the northwesternsub-ocean basin of the South China Sea, with a total length of425 km, so it is a super-large channel depositional system. Thedevelopment of the Central Canyon commenced at the end ofLate Miocene Huangliu Fm. During this period, regressionoccurred widely, greatly promoting the development of thiscanyon. According to studies, the development of this canyonexperienced five stages from the west to the east, furthermore,the canyon was cut by argillaceous channels, and lithologictraps sealed by canyon walls and argillaceous channels inlateral direction and by thick mudstones at its top wereformed. Based on seismic anomalous information andgeophysical attribute features, after analogy analysis wasconducted with the drilled exploratory well Lingshui 17-2, it

Fig. 4. RMS seismic attribute of the ta

was found that they have apparent correlatability, and exhibitgreat similarity (Fig. 4).

In view of the above, there are many favorable traps in thiscanyon. Referring the parameters of the drilled gas reservoirs,it is predicted that the potential hydrocarbon resources only inthe Lingshui section of the Central Canyon can be up to2000 � 108 m3, so it has huge exploration potentiality.

4.2. Exploration inspiration of Lingshui 17-2hydrocarbon reservoir

1) The discovery of Lingshui 17-2 hydrocarbon reservoirand the analysis results of gas components confirmed thedevelopment of coal-measure source rocks in theYacheng Fm of the Lingshui Sag, and the source rocksthere have reached a large-scale hydrocarbon expulsionstage; they also proved that the Lingshui Sag is ahydrocarbon-rich sag, and the exploration potentiality islarge.

2) Based on the analysis on reservoir properties of Lingshui17-2 hydrocarbon reservoir, it revealed that the gravityflow reservoirs in the Huangliu Fm in the Central Canyonare well developed, and they have good physical proper-ties; it is also proved that the positions far way from thecontinental provenances in deep sea also have conditionsfor developing high-quality reservoirs, which removed thedoubt of exploration personnel both at home and abroadon the deep water reservoir bodies.

3) The discovery of Lingshui 17-2 gas field confirmed thatthere is a better correlation between seismic attributeanomaly and hydrocarbon bearing property in the reser-voir bodies of the Huangliu Fm. This provides referencesfor the exploration of other similar targets, and can

rget zones in Lingshui 17-2 trap.

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effectively guide further exploration in the Central Canyonand related areas.

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